Damage resulting from beach erosion is well documented. Hundreds of millions of dollars have been spent to repair such damage; some is irreparable. Beaches provide a buffer to absorb the energy of storm waves, preventing them from destructively crashing into seashore buildings and other property. When the protective beach has been lost due to erosion, the adjacent structures and the land itself are exposed to direct attack by the storm waves. Loss of a beach due to erosion has not only resulted in the destruction of buildings and the loss of land, but has jeopardized the livelihood and existence of resort communities.
At the present time, around 1990, beach replenishment by traditional means attracts more attention and money than the prevention of beach erosion. The process of replenishment replaces or augments the eroded sand beach with large volumes of new sand trucked in or pumped in from offshore areas. Beach replenishment is a stop-gap remedy for an eroded beach but it is not a cure for beach erosion. Periodic maintenance by piling more sand onto the beach is always necessary. Thus the replenishment process is expensive and never-ending.
The well known replenishment of the beach at Miami Beach, Fla., cost a reported $65 million. Virginia Beach, Va., has paid about $1.5 million per year for 30-plus years to haul in replacement sand. Seabright, N.J., is reported spending $158 million to repair its seawall and restore its beaches.
One authority has estimated, as of 1988, that it would probably take 10 years and $300 million to undo the damage already done to Florida's southern beaches and prevent further erosion. These examples emphasize the fact that beach erosion is costly.
Over the years, many means to prevent beach erosion have been proposed and tried. In general, these measures have proven costly and of limited effectiveness over a period of time. Revetments, bulkheads, seawalls, and jetties are major structures. They require heavy materials to be transported and set in place under often-difficult conditions. Cost for the installation of these structures is in the order of hundreds of dollars per linear foot of shoreline.
The destruction and great costs produced by beach erosion clearly demonstrate that there exists a need for more permanent and less costly means to accomplish beach replenishment and the prevention of erosion.
Stimulated by this existing need, many inventions have been conceived and proposed for preventing beach erosion by means other than by the previously mentioned revetments, seawalls, etc. In general, these inventions are objects placed on the beach surface for the purpose of dissipating the energy of the surf to reduce its destructive power, or for the purpose of trapping beach sand that had been raised into suspension by the agitated waters.
It might be generalized that, as to the energy of the surf, most of these inventions are either dissipative or passive. In contrast, it is desirable to utilize the energy of the surf to build up the beach. This is the primary objective of my invention.
Beach erosion--defined by the official SHORE PROTECTION MANUAL--is "the carrying away of beach materials by wave action, tidal currents, or wind". It is evident that beach erosion will be greatly reduced if the "carrying away" action is prevented. "Carrying away" by wave action occurs when beach materials (e.g. sand) have been dislodged from the bottom surface, stirred into suspension, and then swept seaward by the turbulent surf flow.
If the beach materials were not completely dislodged and separated from the bottom surface they would not be raised into suspension subject to being carried away. Thus, beach erosion caused by wave action will be largely prevented if beach materials are retained on the bottom surface and not completely dislodged and free therefrom. This is another of the features of my invention.
One physical principle which is employed in carrying out the objectives of my invention is the optimum use of the kinetic energy of a breaking wave driven against a sloping beach. This kinetic energy nears its maximum as a wave approaches the plunge point. This maximum kinetic energy acts to dislodge and transport sediment as the incoming wave flows up the beach, reaches zero velocity and flows back down the beach to the sea.
The sediment carrying capacity of a body of water moving along an unconsolidated surface such as a sandy beach is primarily a function of the velocity of flow. That is to say, the higher the flow velocity the greater will be the number, weight, and size of solid particles that can be carried by the moving body of water.
In order to make optimum beach-building use of the energy developed by breaking waves, my invention employs means to effect maximum dispersion of sediment into the wave stream during the period of its maximum shoreward velocity, i.e., during the swash portion of the wave cycle, and minimizing sediment dispersion into the waves during the backwash portion of the cycle.
Dispersion of sand and other sediment into the moving surf flow occurs primarily at, or near the plunge point where the plunging surf impacts the sea-bed and dislodges and stirs up sand particles from the bottom.
Beach buildup occurs when the dislodged particles are carried up the beach and deposited as the surf run-up terminates. Reversing its direction, the flow returns back down the beach. Beach erosion occurs when dislodged sediment remains in suspension during the wave flow reversal and is carried out to sea by the backwash.
In ideal conditions the buildup equals or slightly exceeds the erosion and the beach continues to grow, or, at worst, remains of the same extension. But such ideal conditions do not often persist over long periods of time, however, in latitudes subject to relatively violent storms. During a severe storm the sediment-carrying capacity of both the swash and backwash is relatively high, but the swash cycle is of relatively short duration and the potential beach buildup is more than overcome by the longer lasting backwash which under storm conditions has velocity enough to dislodge and move sediment seaward.
An important objective of my invention is to make optimum use of the kinetic energy contained in a moving mass of water, i.e. a wave driven against a sloping beach. This kinetic energy is at or near its maximum as a breaking wave plunges downward at the plunge point. As the wave cycle continues beyond the plunge point, the kinetic energy dislodges and transports particles and is progressively expended reaching zero at a turn-around point where the wave ends its shoreward movement, drops its load of sediment, and drains back down the beach to the sea.
During high surf conditions there is usually an unfavorable imbalance of in-shore and off-shore sediment transport because under these conditions the backwash velocity is great enough to dislodge and transport sediment and is of longer duration than the swash.
My invention corrects this imbalance by restricting the in-flow to a relatively high velocity current immediately adjacent the sea-bed where the water is comparatively rich in dislodged sediment and by causing the out- flow or backwash to drain away in a relatively slowly moving surface-adjacent stream out of direct contact with a part of the bottom underlying the surf zone.
To achieve the objectives of my invention I also employ the operating principle that when a fluid flows over an unconsolidated surface composed of sediment particles such as sand, the total mass of particles moved is a function of the flow conditions along the surface, the amount of surface area exposed to the flow, and the duration of the flow. Therefore, to implement this principle in building up a beach, an embodiment of this invention should have two major operational features:
1. Conditions in the surf-zone must be such that the kinetic energy of the in-flow is employed to cause greater bottom scouring and sediment movement than is caused by the kinetic energy of the out-flow. PA1 2. Such conditions should expose more area of the surf zone under-surface to the scouring in-flow than to the backwash.
These two operating features cause the surf in-flow to move more sediment, or sand, than does the surf out-flow. By this selective utilization of the energy of the surf in-flow, the resulting net movement of sand picked up from the sea-bed, is toward the shore, terminating with deposition on the beach.
These operating principles are implemented by use of a thin flexible sheet, herein designated "flow-control sheet" or FCS. It is fixed in its horizontal location over a portion of the sea-bed under the surf zone. The FCS is free to move vertically within limits, and has a buoyed seaward edge which causes the edge to be lifted away from the sea-bed by each surf in-flow. After the FCS edge has been raised, all or part of the surf in-flow passes beneath the sheet, applying its energy to dislodging and moving particles of sand along the bottom toward the shore.
When the in-flow stops, the weight of the shoreward edge of the flow-control sheet sinks it to rest upon the bottom. The ensuing out-flow then passes over the FCS, and out of contact with the sea-bed. Hence, the in-flow or swash is always in more intimate contact with the sea-bed than is the out-flow or backwash.
An important benefit of the cyclic flow diversion just described is that "bed load tranport" in a seaward direction is greatly reduced since loose sand remaining after the in-flow, that might otherwise be rolled out to sea is immobilized by the FCS until the next in-flow lifts the FCS and moves the loose sand shoreward.
To summarize, the above-described objectives are accomplished by the use of a flexible FCS suspended on or near the under-surface of the surf zone. The FCS is tethered in place by anchor lines leading from the seaward and shore-adjacent edges. The sheet is also caused to slope upwardly in a seaward direction by incorporating elongated floats in the seaward edge and a flexible elongated weight such as a chain in the shore-adjacent edge.
The tethers may be adjustable in length so as to make possible the movement of the entire flow-control sheet, seaward or shoreward, as wind and wave conditions dictate.